Pneumocystis carinii pneumonia (PCP) is the most common opportunistic infection reported in HIV-infected children. While trimethoprim- sulfamethoxazole (TMP/SMX) remains the drug of choice for PCP prophylaxis, drug sensitivity may limit its effective use; in addition, the efficacy of the most common alternative therapies-including dapsone and pentamidine-has yet to be determined in infants and children. Atovaquone (AT), an oral medication requiring only once daily administration, has demonstrated activity against P. carinii and Toxoplasma gondii, and has been shown to be both safe and with no defined adverse or toxic effects. Primary objectives of this multi-centered, two armed, double blind, placebo controlled study will be to compare Trimethoprim/Sulfamethoxa (TMP/SMX) and Atovaquone/Azithromycin (AT/AZ) in the prevention of serious bacterial infections; determine and compare long-term safety and tolerance; and assess the liklihood of pharmacokinetic interaction between atovaquone and azithromycin in a subset of two to 18 year old patients.